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Fuelling 101 · 7 min readWhy under-eating wrecks performance
The hidden cost of training hard on too little fuel, and how to fix it.
The number that matters is not calories
Energy availability is what is left for your body to run on once training has taken its share. It is measured against fat-free mass, and it is a different question from whether you are in a calorie deficit: you can eat what looks like plenty and still leave your physiology short, simply because the session took more than you replaced.
The research threshold most often cited is around 30 kcal per kilogram of fat-free mass per day. Below it, the body starts making economies. Above roughly 45, most systems have what they need. The band between is where a lot of women who train hard actually live, without ever suspecting it, because nothing hurts and the scales look fine.
kcal per kg of fat-free mass, per day
The band most athletes should train in
Energy usually moves first
What the body switches off first
The economies are not random. Reproductive function is expensive and not immediately necessary, so it goes early: cycles get longer, lighter, then stop. Bone turnover tips towards resorption. Thyroid output drops, and with it your resting metabolic rate, which is why the deficit stops working and eating less stops helping.
Immune function, mood, sleep quality and the ability to actually adapt to training all sit downstream of the same thing. The frustrating part is that the symptom set reads like ordinary tiredness, so the usual response is to train harder and eat less, which is precisely the wrong direction.
An illustrative summary of the pattern reported in the low-energy-availability literature. The thresholds are population-level reference points, not a personal cut-off: the number your own body needs depends on your training load, your history and your health.
How to correct it
Correction is rarely dramatic. It is usually more carbohydrate around training, a real breakfast, and a genuine rest day. Most women I work with need to add food rather than subtract it, and most feel the difference within a fortnight — energy first, then sleep, then training quality.
The markers worth watching while you do it are ferritin, thyroid function, and, if your cycle has changed, a hormone panel. They tell you whether the correction is landing rather than leaving you to guess from how you feel on any given Tuesday.
- 01
Eat around the session, not only at the end of the day
Timing - 02
Put carbohydrate back before you touch anything else
First move - 03
Take the rest day as written, not as an optional extra
Recovery
This is general nutrition education, not medical advice or a diagnosis. Persistent fatigue, a cycle that has changed or stopped, or a stress fracture all warrant a conversation with your GP. Full health disclaimer →
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